a) Field of the Invention
The invention is directed to a method for determining visibility, amount of precipitation and type of precipitation, and to a combined visibility and precipitation measuring instrument, particularly for distinguishing between different types of precipitation and amounts of precipitation.
b) Factors Relating to the Invention
Measurement of environmental parameters and weather parameters is growing in importance. In order to replace former visual observations with automatic detection of weather phenomena and, accordingly, in order gradually to enable a complete automation of weather stations, the availability of measurement instruments capable of determining as many of the required parameters as possible is required.
Atmospheric turbidity is brought about by aerosols whose type and concentration determine visibility. In this connection, there are two physical effects that are important in determining visibility with optical measuring instruments. Irradiation of a measurement volume by a light source produces light scattering on the one hand and light absorption on the other hand.
Both effects are detected by a transmissometer arrangement because the latter evaluates what percentage of the emitted light reaches the location of the receiver. The reception signal is influenced by scattering at the aerosols as well as by absorption by the aerosols. Both phenomena are covered under the concept of extinction, wherein the absorption may be disregarded in favor of scattering. Transmissometers measure the atmospheric extinction coefficient directly. The visibility can then be determined by definition by way of the relationship described by Koschmieder.
Apart from the large air volume utilized for determining visibility, the advantage of transmissometers consists in this definite relationship between transmission and visibility. The quality of the measurement performance of transmissometers is generally very highly rated.
Nevertheless, there are reasons in favor of using a simplified measurement concept such as that offered by scattered light measurement devices:
greater visibility measurement range;
possibility of determining the type and intensity of precipitation;
low initial costs;
reduced space requirement; the typical distance of the transmitter and scattered light receiver from the measurement volume is less than 1 m;
no aligning or adjusting labor is required;
reduced influence of contaminated device plates on measuring performance;
simple calibration; and
low maintenance costs.
c) Description of the Related Art
In scattered light measurement devices such as those described in DE-OS 21 21 088, a receiver is positioned at an appropriate angle to the light source. The light component scattered by the aerosols and precipitation particles in the measurement volume at this angle is determined. In this case, knowledge of the relationship between the optical scattering function and the atmospheric extinction coefficient is required for evaluating the reception signal.
Differences in the scattering behavior between the various particles cannot be taken into account adequately, or at all, in conventional scattered light measuring instruments by measuring either at a selected scattering angle or over the largest possible scattering angle area.
The conventional devices realize methods that are based on the assumption that the utilized scattering angles in the light wavelength used for the measurement can be taken as representative for all occurring phenomena.
In this connection, a forward scattering angle of approximately 30° to 40° has proven to be representative for a large number of types, mixtures and concentrations of aerosols based on various simulations of atmospheric models and practical tests and comparisons for the visible wavelength range. In general, the theory of scattering proposed by Mie applies for this order of magnitude of particles with much greater diameters than the light wavelength. However, different assessments of the measured scattering signal are also necessary in this case because there are significant differences in the scattering behavior of mist and fog.
There are very many greater influences when considering precipitation. Drizzle droplets and raindrops, hail, soft hail or snow pellets, and snowflakes result in entirely different scattering behavior for which it is no longer possible to carry out the same evaluations of the measured scattering signal as those for mist and fog. Further, these particles do not remain in a quasi-stationary spatial position; rather, because of their size, they undergo a movement in the direction of the ground so that the period during which they remain in the measurement volume is limited.